The GPLA complex is the major histocompatibility complex of the guinea pig, and thus regulates several aspects of the immune response. Two sets of cell membrane glycoproteins, the classic histocompatibility GPLA-B and -S antigens, and the Ia antigens appear to have central roles in the immune responses controlled by the GPLA complex. The research proposed utilizes a chemical approach to determine how the structure of the GPLA-B/S and Ia antigens influences their function. Radiolabeled, detergent solubilized, partially purified antigen molecules will be isolated by indirect immunoprecipitation techniques, and separated into their component chains. These chains will then be subjected to tryptic and other peptide mapping techniques and compared. Attempts will be made to assign alleles. Partial N-terminal amino acid sequencing will be done using the micro sequencing technique. Ia antigens derived from subpopulations of immunocompetent cells will also be compared by these techniques for identity or differences. The role of carbohydrate in the function of the antigens will be assessed using the glycosylation inhibitor, tunicamycin. Membrane associations of various Ia molecules on the cell surface will be examined using reversible cross-linking reagents of various lengths.